Effects of Lipid Emulsion on the Pharmacokinetic and Pharmacodynamic Properties of Metoprolol.

Overdose or poisonings with cardioactive drugs can have serious consequences. In recent years, intravenous lipid emulsion has emerged as a possible treatment option in otherwise treatment-resistant cardiovascular collapse caused by poisonings with cardio-active drugs.

Experimental evidence obtained from animal studies has been indicating a beneficial effect of intravenous lipid emulsion in the treatment of poisoning with various cardio-toxic medications. Based on these findings, the first reported cases on the use of intravenous lipid emulsion in the treatment of human cardiotoxicity caused by poisonings with local anesthetics were published in 2006. Subsequently, a steadily increasing number of case reports concerning the use of intravenous lipid emulsion in resuscitation and treatment of various medications poisonings have been published. Often patients had either cardiac arrest or severe circulatory failure treated according to guidelines for advanced life support prior to lipid emulsion therapy. It is noteworthy that a common observation following bolus infusion of lipid emulsion has been a rapid hemodynamic stabilization of the patient.

Despite an increasing use of intravenous lipid emulsion in the treatment of the poisoned patient, the mechanism behind lipid rescue has not been elucidated. The most widely accepted hypothesis, the "lipid sink/sponge" model, suggests that intravenous lipid emulsion entraps xenobiotics intravascularly, thereby preventing them from reaching sites of toxicity. Additionally, intravenous lipid emulsion may redistribute xenobiotics to areas of higher lipid content. However, other mechanisms of actions of lipid emulsion, supported by observations from animal experiments, are vasoconstrictive and cardio-tonic effects. These effects could be secondary to direct activation of sodium, potassium or calcium channels in the myocardium, or alternatively fatty acid-induced modulation of the metabolic properties of mitochondria. Both mechanisms could result in hemodynamic stabilization. The potential beneficial effects of lipid emulsion on hemodynamic instability beyond the lipid sink have led to the notion that intravenous lipid emulsion could be valuable in the treatment of poisonings with non-lipophilic xenobiotics.

At present, it remains however unclear to what extent the evidence concerning resuscitation of the poisoned patient with lipid emulsion may reflect publication bias. To our knowledge, only one controlled human trial has been conducted at present. In a randomized crossover study, Litonius et al. investigated the effects of lipid emulsion on plasma concentrations of bupivacaine in eight healthy subjects. It was found that lipid emulsion lowered the total plasma concentrations of bupivacaine. This was attributed to an altered distribution and contradicted so the above hypothesis of a lipid sink-mechanism as the fraction of non-lipid bound bupivacaine was unchanged. The mechanism by which lipid emulsion may attenuate the effects of cardio toxic xenobiotics must therefore still be regarded as undecided. This lack of evidence calls for further human studies in order to elucidate the pharmacokinetic and pharmacodynamic consequences of intravenous lipid emulsion. The purpose of this double blind, randomized placebo-controlled crossover clinical trial is to investigate the effects of intravenous lipid emulsion on the pharmacokinetic and pharmacodynamic properties of the adrenoceptor antagonist metoprolol in a human model of beta blocker overdose.

The study includes a total of five visits; a screening visit and four trial days. At the screening visit, anthropometric data (weight, height, blood pressure and pulse) is measured. Additionally, blood samples are collected in accordance with exclusion criteria. A spot urine sample measuring the albumin/creatinine ratio is collected and an electrocardiogram (ECG) is recorded to verify normality of heart rhythm and electrical impulses. In addition, an investigator carries out a clinical examination. Based on the clinical examination, urine and blood tests and ECG measurement, the investigator assesses whether the trial participant meet all inclusion criteria and no exclusion criteria.

After screening and inclusion, participants will be invited to four trial days at the trial site. On each day participants are required to be fasting for 10 hours (including water, coffee and tobacco). A peripheral venous line is inserted into each antecubital vein. An arterial catheter connected to a pressure transducer is inserted into the radial artery in the wrist. In randomized order, one of the four interventions are performed (see below). A standard 12 lead ECG is placed on the participant as well as a 5 lead ECG connected to a computer.

At T=0, metoprolol intravenous solution (0.5 mg metoprolol/ml as metoprolol tartrate) or placebo is administered as an intravenous bolus injection. Continuous infusion of metoprolol/placebo is then administered until T=30 minutes. Infusion is halted if heart rate drops below 35 bpm or systolic blood pressure drops below 80 mm Hg, or the participant experiences subjective side effects. Infusion stops at T=30 minutes. Intravenous lipid emulsion (Intralipid 20 %) or saline solution is shortly thereafter administered as an intravenous bolus infusion (1.5 ml/kg) followed by a continuous infusion (infusion rate: 0.25 ml/kg/min). Lipid emulsion/dummy infusion is stopped at T = 30 minutes. One gram of paracetamol administered as a disintegrating tablet dissolved in 50 ml of water is given per os shortly before study start on each day.

Repeated ECG's are recorded and blood is drawn for measurements of routine biochemistry parameters and serum concentrations of metoprolol and paracetamol. A drop of blood is used to test glucose levels using a blood glucose meter. Cardiovascular parameters (heart rate, blood pressure, pulse contour curve/arterial pressure wave) are recorded via the arterial catheter and pressure transducer connected to a computer. The participant is closely monitored on site until T=120 minutes.